1. Field of the Invention
The present invention relates generally to communication systems and more particularly to a method for comprehensive estimation of independent channel parameters.
2. Prior Art
In a multi-path environment of a wireless system, there is a need to evaluate the channel conditions, and provide a set of parameters that give an indication of the quality of the channel. Each of the channels that may be used is checked and then those having the best transmission results may be selected for communication. The understanding of the channel quality is particularly important in order to ensure the correct operation of a wireless system using orthogonal frequency division multiplexing (OFDM). In this case, multiple frequencies are multiplexed in a relatively narrow band, collectively forming a wide band transmission link. Environmental impact on the transmitted signal raises the need for timing synchronization, used to position the signal in an optimum sampling window and further ensure phase alignment of the signals. It is therefore essential to understand the channel characteristics so as to provide the necessary parameters to the wireless system for the necessary adjustments.
Roy III et al. show in U.S. Pat. No. 4,750,147 a technique based on maximum likelihood entropy principles, or subspaces, for the estimation of sinusoid signal parameters. Therefore, this technique cannot be directly applied to the estimation of signal parameters in an OFDM system. This is because the technique is directed toward extracting all of the signal transmission channel parameters. It is well-known in the art that there is insufficient data to realize such an approach. Even if it is assumed to be possible, extracting all the signal transmission channel parameters would naturally require a significant amount of data from the measurements. This would necessarily lead to a complex implementation which may further lead to a slower than desirable response time.
In U.S. Pat. No. 6,587,526, Li et al. suggest a different approach, noting that it is impractical to handle all the possible channel parameters in an OFDM system. Li et al. suggest to overcome prior art deficiencies by employing an arrangement and a method that does not require the estimation of all the signal transmission channel parameters. More specifically, a rotational invariance arrangement or technique that utilizes either spectral, and/or temporal, and/or spatial diversity present in the wireless system, is used to generate a single prescribed parameter of the mobile signal transmission channel. In one of the embodiments of the invention disclosed by Li et al. the single prescribed parameter of the signal transmission channel is the delay that is substantially constant over the diversity, while the gain of each signal transmission channel path may vary.
There are many parameters in an OFDM system to be uniquely identified. The different operation modes, for example for a HiperLAN/2 standard, are shown with reference to FIG. 5. The limitations of the prior art go between complex systems that require the estimation of all the parameters, to simpler solutions that select a single prescribed parameter for estimation purposes. There are well-known coarse, simplified, and exact channel estimation methods, each employed separately from each other, and without consideration of when one should be used over the other as channel estimation takes place. A coarse channel estimation is generally referred to as a case where the channel estimation is not very accurate and has a low computational budget, however, it must be recalculated for each time there is need for a channel estimation. Exemplary and non-limiting discussions of coarse channel estimation may be found in “Channel Quality Estimation and Rate adaptation for Cellular Mobile Radio”, by K. Balachandran, S. et al., IEEE Journal on Selected Areas in Communications, vol. 17, no. 7, July 1999, pp. 1244-1256, and “A Novel OFDM Transmission Scheme with Length Adaptive Cyclic Prefix”, by Zahng, Z. and Lai, L., Journal of Zhejiang University SCIENCE, 2004, Vol. 5(11), pp. 1336-1342. Exemplary and non-limiting discussions of simplified channel estimation may be found in “A Reduced Complexity Channel Estimation for OFDM Systems with Transmit Diversity in mobile Wireless Channels”, by Minn, H. et al., IEEE Transactions on Communications, vol. 50, no 5, May 2002, pp. 799-807, “An Investigation into Time-Domain Approach for OFDM Channel Estimation Approach”, by Minn, H. and Bhargava, V., IEEE Transactions on Broadcasting, vol. 46, no 4, December 2000, pp. 240-248, and “A Low-Complexity ML Channel Estimator for OFDM”, by Luc Deneire et al., IEEE Transactions on Communications, Vol. 51, No. 2, February 2003, pp. 135-140. Exemplary and non-limiting discussions of exact parametric channel estimation may be found in “Estimation of Multipath Parameters in Wireless Communications”, by Vanderveen, M. C. et al., IEEE Transactions on Signal Processing, vol. 46, no. 3, March 1998, pp. 682-691, and “Multichannel MLSE Equalizer with Parametric FIR Channel Identification”, by Chen, J. et al., IEEE Transactions on Vehicular Technology, vol. 48, no. 6, November 1999, pp. 1923-1935. Exemplary and non-limiting discussions of exact detailed channel estimation may be found in “A Two-Stage Receiver for Co-channel Interference Cancellation in Space-Time Block-Coded Systems over Frequency Selective Channels”, by de Almeida, A. L. F et al., XX Simposio Brasileiro De Telecomunicaciones—05-08 De Outubro De 2003, Rio De Janeiro, “Channel Estimation Using Pilot Tones in OFDM Systems”, by Yeh, C. and Lin, Y., IEEE Transactions on Broadcasting, vol. 45, no. 4, December 1999, pp. 400-409, “A Comparison of Pilot-Aided Channel Estimation Methods for OFDM Systems”, by Morelli, M. and Mengali, U., IEEE Transactions on Signal Processing, vol. 49, no. 12, December 2001, pp. 3065-3073, “Pilot Tone Selection for Channel Estimation in a Mobile OFDM System”, by Negi, R. and Cioffi, J., IEEE Transaction on Consumer Electronics, vol. 44, no. 3, August 1998, pp. 1122-1128, and “On channel estimation in OFDM systems”, by Edfors, O. et al., proc. of Vehicular Technology Conf., 1995, pp. 815-819. Exemplary and non-limiting discussions of discrete Fourier transform (DFT) based exact channel estimation may be found in “Analysis of DFT-Based Channel Estimators for OFDM”, by Van de Beek, J. J. et al., Kluwer Academic Publishers, Wireless Personal Communications 12, 2000, pp. 55-70. Other known prior art deals with specific aspects of channel estimation, but none provide a comprehensive channel estimation solution.
The disclosures of the foregoing references are hereby incorporated herein by reference.
There is therefore a need in the art to provide a method to allow a flexible and comprehensive solution that estimates the channel parameters based on the specific characteristics of a channel. It would be further advantageous if such a solution would allow for various levels of parameter extractions based on the specific needs of a wireless channel.